Refactor and enhance Download Manager application

- Added new services: IContentDialogService, ISnackbarService
- Replaced DashboardPage and DashboardViewModel with DownloadingPage, DownloadingPageViewModel, WaitingPage, and WaitingPageViewModel
- Introduced AddNewTaskPage and AddNewTaskViewModel
- Replaced DownloadServiceManger with DownloadManagerService
- Updated WPF-UI package to 3.0.4 and added HtmlAgilityPack 1.11.61
- Refactored DownloadWorker to use async methods and added retry logic
- Converted DownloadItemData to partial class with ObservableProperty attributes
- Changed initial navigation page to DownloadingPage
- Updated MainWindow navigation items and title
- Added new utility classes: DoubleUtilities, MD5Utilities, StringExtension
- Added new XAML pages and ViewModels for task management and display
- Removed obsolete DashboardPage and related files

Views/Pages/WaitingPage.xaml.cs

@@ -0,0 +1,19 @@
+using DownloadManager.ViewModels.Pages;
+using Wpf.Ui.Controls;
+
+namespace DownloadManager.Views.Pages;
+public partial class WaitingPage : INavigableView<WaitingPageViewModel>
+{
+    public WaitingPageViewModel ViewModel
+    {
+        get;
+    }
+
+    public WaitingPage(WaitingPageViewModel viewModel)
+    {
+        ViewModel = viewModel;
+        DataContext = this;
+
+        InitializeComponent();
+    }
+}